March
4, 1999

An experiment to study black holes, neutron stars and other exotic
celestial objects was launched Feb. 23 from Vandenberg Air Force Base in
California, with the goal of learning more about matter in its most extreme
states.

A collaboration of astrophysicists the Department of Defense's Naval
Research Laboratory (NRL) and particle physicists from SLAC and have built
an x-ray detector that will be used in one of nine primary experiments
on board the Air Force Space Test Program's Advanced Research and Global
Observation Satellite (ARGOS). The detector is called the Unconventional
Stellar Aspect (USA) experiment. The Department of Defense and the Department
of Energy have provided funds for USA.

The ARGOS Satellite in orbit with USA pointing at the
viewer. In the lower corner of the picture is former SLAC/Stanford graduate
student, now Dr. John Hanson, sitting in the USA support structure, which
he helped design and build as part of his Ph.D thesis.

ARGOS was launched by a Delta II rocket that placed the satellite into
a polar orbit where it is expected to operate for at least three years.

The USA experiment was designed to observe bright x-ray sources, mostly
binary star systems, including a black hole, a neutron star, or a white
dwarf, orbiting with a more typical star. In neutron stars, gravity has
compressed matter down to densities larger than those found in the nucleus
of an atom. In all of these types of binary systems, extraordinarily strong,
relativistic gravitational forces and enormous magnetic fields act in concert
to produce dramatic phenomena not observable from Earth-based laboratories.

In addition to providing valuable new information for astrophysicists
and particle physicists, USA has been designed to make significant contributions
to applied science, environmental science, and engineering research. It
will use x-ray sources to test new approaches to satellite navigation and
to conduct the first tomographic survey of Earth's atmosphere. It will
also test new concepts for making spacecraft computers more reliable, an
approach called fault-tolerant computing.

"This

Liftoff of the Boeing Delta II rocket with
the ARGOS satellite on board, in the early morning, on February 23, 1999
from Vandenberg Air Force Base. (See also the QuickTime
movie.)

is the first time that SLAC has been directly involved in a space-based
experiment," said SLAC Professor Elliott Bloom. "USA gives us the chance
to do in-depth particle astrophysics research on many different black hole
and neutron star systems in our galaxy. The collaboration between particle
physicists at SLAC and astrophysicists at the Naval Research Laboratory,
which began in 1991, gives us a unique approach to studying these systems."
The instrument's observations of binary star systems that contain a black
hole or a neutron star should provide new information about the behavior
of relativistic gravity near these compact objects. The instrument will
also probe regions of extreme temperature and density in order to test
the standard model of particle physics and, perhaps, find evidence for
new types of matter.

To search for interesting new physics, USA can observe variations in
x-ray intensity with a time resolution of less than 100 microseconds. The
experiment consists of two large area x-ray sensors on a gimbaled mounting.
The sensors are collimated proportional counters, sensitive to X-rays between
1 and 10 angstroms, and with a field-of-view of 1.5 degrees. SLAC's main
role in the experiment was to build and test the collimators, and construct
the mounting. SLAC scientists and graduate students also helped to debug
and test the readout electronics. They are currently busy trying to understand
the calibration of the instrument, and preparing to analyze the large quantities
of scientific data that are expected shortly. USA has an average telemetry
rate of about 40 Kbytes per second - small by particle physics standards
- but equal to that of the largest space-based experiments currently in
orbit.

The Delta II Rocket shortly after liftoff, prior to separation.
(See also the QuickTime
movie.)

To accomplish its mission, USA will observe a small number of preselected
targets and re-measure the x-rays from these targets repeatedly. The scientists
anticipate that each of approximately 30 bright sources will be observed
several times during the first month of operation, which should begin about
May 1, 1999.

As part of its engineering research goals the USA experimental team
will test new concepts for fault-tolerant computing in space. To achieve
this goal, USA carries a two-computer testbed that consists of a military
radiation-hardened processor side by side with a commercial off-the-shelf
processor. The testbed will allow scientists to determine the effectiveness
of advanced fault-tolerant software algorithms designed to allow the processor
to continue operating even when damaged.

Assembly of the ARGOS satellite onto the Delta II rocket
inside the Mobile Launch Support Complex, located on the launch pad at
Vandenberg Air Force base near Santa Maria, California. (See larger
photo for more detail.)

The principal investigator for the USA experiment is Kent S. Wood from
NRL. Wood is a Stanford University Alumnus. Michael N. Lovellette, also
from NRL, is the project scientist. Michael Wolff and Paul Ray of NRL have
also been prominent in the construction phase of USA. Bloom is the lead
co-investigator for the Stanford part of the collaboration and the group
leader of SLAC's Particle
Astrophysics group. Other Stanford members of the collaboration include
Gary Godfrey of SLAC, Stanford physics Professor Peter Michelson, Visiting
Professor Lynn Cominsky (Sonoma State University) and a growing number
of SLAC/Stanford graduate students (Ganya Shabad, Daniel Engovatov, Pablo
Saz-Parkinson, and Kaice Reilly). There is also a broader membership in
the USA collaboration poised to analyze the prolific data soon to be beamed
down from the spacecraft. In addition to those at NRL, Stanford and Sonoma
State, these include scientists at University of Calgary, Massachusetts
Institute of Technology, and NASA Ames Research Center.

The USA, installed on top of the ARGOS satellite. Michael
Lovellette, USA Project Scientist from the Naval Research Lab, 2nd from
right.